Methods for manufacturing semiconductor devices simultaneously to implement low-voltage and high-voltage devices in a single chip. In one example embodiment, a method includes various acts. An isolation layer is formed on a wafer. A gate oxide layer and a lower gate poly are sequentially formed on a first low-voltage transistor region. A first poly oxide layer is formed. A nitride layer is formed on the first poly oxide layer. The nitride layer and the first poly oxide layer are etched. A field oxide layer is formed by selectively oxidizing portions exposed by the etching. A second poly oxide layer is formed. Gate patterns of each transistor region are completed by vapor-depositing an upper gate poly on a high-voltage transistor region, the first low-voltage transistor region and a second low-voltage transistor region. A source and drain region are formed.